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AT48802

Spread-Spectrum Signal Processor Integrated Circuit

Atmel Corporation 

PN Code Generation

The AT48802 contains two independently programmable

pseudo-random noise (PN) generators. One is used for

transmit and the other is for receive. They are 13-stage

linear feedback shift registers clocked at f(master clock) /

16, or the “chip rate”, normally 960 kHz (based on a 15.36

MHz master clock). Each can be programmed to operate

with lengths of 7 to 13-stages PN (8,192 bit code se-

quence length). These lengths are actually linear maximal

lengths plus one to simplify the internal circuitry. The long

code length has the benefit of having many different maxi-

mal-length codes available for co-location operation in

similar spread-spectrum equipment with minimum mutual

interference, thus allowing efficient use of frequency chan-

nels. For example, there are over 600 maximal-length se-

quences available for R13 PN, and over 300 for R11 PN.

Each maximal-length code can be considered a unique

user channel.

The “Mask” bit in each PN register controls the counter

sequence by setting feedback tap weights to either 0 or 1.

The Transmit PN Generator (Tx PN) output and the Re-

ceive PN Generator (Rx PN) output are time division mul-

tiplexed precisely by the 50% duty-cycle TX PWR signal.

That is, during the transmit cycle, only the Tx PN codes

are outputted at the TX/RX PN pin. Conversely during the

receive cycle, only the Rx PN codes are outputted. There

is no prohibition against using the same code for transmit

and receive. The shift register taps are set at R2 b0-4 and

R1 b0-7 for receive, and R4 b0-4 and R3 b0-7 for transmit.

For definition purposes the end of the link which is initiat-

ing the link is the MASTER, and the end which is respond-

ing is the SLAVE. This means, e.g., for a cordless phone,

if you are calling out then the handset becomes the MAS-

TER and the base station is the SLAVE. If someone is

Figure 2. PN Coupling for Spectral Control

AT48802

VCC

470

TX_RX_PN

0.1 uF

To RF Circuit

R is chosen to make the immediate

R

value of PN output equal to VCC /2

calling you, then the base station is the MASTER (be-

cause it is initiating the radio link) and the handset is the

SLAVE. This function is set at R0 b6.

If the chip is the MASTER, then the transmit PN generator

is clocked from the clock generator and the receive PN

generator is clocked from the chip phase control (through

the tau-dither generator). If the chip is the SLAVE, then

both PN generators are clocked from the chip phase con-

trol. Therefore the MASTER transmit has independent tim-

ing and the SLAVE locks both PN generators, via the chip

phase control, to the receive signal. Finally, the MASTER

receive PN uses the chip phase control to lock to its re-

ceived signal from the SLAVE. In this way one can see the

outline of an acquisition process.

The AT48802 PN spectral control feature enables the ra-

dio frequency transmit spectrum to easily meet the FCC

requirement that out-of-band energy in a 100 kHz band-

width be at least 20dB below in-band maximum energy in

the same bandwidth. By this means one can achieve more

spreading and more widely spaced frequency channels

with less output filtering and still meet the requirements.

The TX RX PN output is three-stated for one MCLK (mas-

ter clock, the 15.36 MHz input ) at each transition. By

means of external pull resistors, this makes the PN volt-

age waveform rest at VCC/2 for 60 ns on every transition.

The objective is that the RF transmit power should go to

zero during these periods. This introduces a spectral

notch at 7.5 MHz on each side of center. If this waveform

is faithfully preserved by the spreading mixer and sub-

sequent amplifiers then the RF transmit spectrum will

have nulls near ±7.5 MHz. This reduces the normal PN

lobes which might otherwise exceed allowed amplitude. A

particular application may or may not need this feature; for

example, if only one frequency channel is being used, and

it is in the center of the band, then depending on the output

filter one may not have this problem. In such a case a sim-

ple lowpass filter may be used from the PN generator out-

put to the RF module PN input.

A force-load function is provided for initializing the PN gen-

erator to ensure the transmit and receive PN generator

coefficients can be loaded into the counters without lock-

ing up during the first-time loading after a power up cycle.

This is common among multiple feedback PN counters.

The force-load bit can be set by a logic 1 to the FLOAD bit

in the control register (Register 0, bit 1).

The Transmit PN and the Receive PN counters can be

synchronized by asserting a logic 1 to the PN RESET bit

in the control register (Register 0, bit 0). The PN OUT

function at R0 b5 turns on the PN when set.

2-6

AT48802

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